Steering Pt Cluster Dimensionality via the Surface Oxidation State of CeO₂(111) Thin Films

Ceria has recently regained attention in catalysis research, thanks to its ability to reversibly form and redisperse supported, catalytically active Pt clusters through control of its surface morphology and oxidation state. In the present article, we systematically and independently tune these parameters during CeO₂(111) film synthesis to investigate their influence on the dimensionality (2D vs 3D) and sintering behavior of size-selected Pt₂₀ clusters. We present recipes for atomically flat CeO₂(111) islands and closed films with a thickness of up to 18 monolayers, grown on Rh(111), and characterize them by means of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and low-energy electron diffraction (LEED). Remarkably, XRD and LEED reveal an epitaxially grown, crystalline, and relaxed closed film of a single domain, with cube-on-cube alignment. Bulk or exclusive surface reduction is achieved by ultra-high vacuum annealing or room temperature CH₃OH dosing and annealing cycles, respectively. The methanol procedure forms oxygen vacancies only in the surface without reducing the deeper layers of the film or introducing roughening. From STM images, we extract detailed height distributions and coverages of Pt₂₀ clusters and find that Ostwald ripening already sets in around 600 K on both, fully oxidized and surface-reduced ceria, without any indication for cluster diffusion and coalescence. XPS shows that atom detachment during sintering leads to the intermediate formation of Pt²⁺ species on oxidized ceria, in line with the redispersed single atoms at step edges observed in the literature. Strikingly, while the clusters appear similarly upon deposition on both supports, they show a distinct temperature-dependent dimensionality upon annealing: Exclusively 3D clusters form on the oxidized support, while most clusters on the reduced support adopt a flat, 2D geometry upon sintering, stabilized by O vacancies.